In the past, control apparatuses of servo motors have been used in machine tools and industrial machines. In particular, the control apparatus of a servo motor using feedback control performs control so that the differences between the position, speed, current, and other command values and actual values always become zero, so the machine position, motor speed, etc. are accurately controlled and therefore is being broadly employed in general CNC machine tools etc.
On the other hand, the mechanical system comprised of a servo motor and a driven part driven by this servo motor is a physical dynamic system having elasticity, friction, mass, etc. When the rigidity or attenuation characteristics are low, sometimes there is mechanical resonance.
Therefore, control apparatuses for motors for suppressing vibration due to such mechanical resonance are being proposed. For example, Japanese Patent No. 2504307 discloses a speed control apparatus of an electric motor provided with a filter to be inserted in a torque command in a speed feedback control loop in a mechanical system comprised of an electric motor and a load driven by this electric motor, where this filter limits the passage of a frequency component due to mechanical resonance between the electric motor and load included in the speed detection value of the mechanical system.
Further, Japanese Patent Publication (A) No. 2004-272883 discloses a servo control apparatus provided with a mechanical property compensating unit attenuating a predetermined frequency component corresponding to a characteristic of the machine to be driven from a position command signal so as to calculate the position, speed, and torque feedforward signals and a feedback compensating unit driving the machine to be driven in accordance with the position, speed, and torque feedforward signals calculated by the mechanical property compensating unit. In this control apparatus of a servo motor, the mechanical property compensating unit is provided with a torque command calculating unit attenuating the resonance frequency component of the machine to be driven and calculating a torque feedforward signal.
In the speed control apparatus of an electric motor described in Japanese Patent No. 2504307, the mechanical resonance frequency of the mechanical system is in a frequency region higher than the frequency band of the gain of the transfer function of this speed control apparatus.
Further, in the servo control apparatus described in Japanese Patent Publication (A) No. 2004-272883, a filter for attenuating the mechanical resonance frequency component of the machine to be driven is provided in the calculating unit of the torque command signal of a motor for calculating the torque feedforward signal.
In this way, when the mechanical resonance frequency of the mechanical system is relatively high, as shown in Japanese Patent No. 2504307 and Japanese Patent Publication (A) No. 2004-272883, it is possible to suppress vibration due to mechanical resonance by providing a filter for the torque command signal of the servo motor.
However, sometimes the mechanical resonance frequency of a mechanical system is a low frequency overlapping the frequency band of the gain of a transfer function of the control apparatus of a servo motor, that is, a low frequency where the speed control can sufficiently react. In this case, in the control apparatus of Japanese Patent No. 2504307 or Japanese Patent Publication (A) No. 2004-272883, either the vibration due to mechanical resonance cannot be sufficiently suppressed or the precision of the position to be controlled ends up falling, so the precision of work of the machine tool etc. is liable to fall.
For example, in a machine tool operating with a plurality of axes in cooperation, mechanical resonance of such a low frequency occurs. In this case, if applying the filter of Japanese Patent No. 2504307 or Japanese Patent Publication (A) No. 2004-272883 to the position commands of the axes and the speed commands of the axes arising from the same, the path resulting from the combination of the paths of the plurality of axes may greatly deviate from the composite path of the position commands.
Further, in a machine tool, when applying a filter to the feed shaft where low frequency mechanical resonance occurs so as to suppress such vibration, the work results may deteriorate due to the effect of the control delay of the operating shafts at locations of sharp angle operation and locations of high acceleration. In this way, if simply filtering the position command or speed command, sometimes speed commands required for acceleration or deceleration of the motor are not sufficiently output. As a result, the work precision of machine tools etc. is liable to deteriorate.
Further, if lowering the frequency band of the gain of the transfer function in the control apparatus of a servo motor below the frequency of the mechanical resonance to avoid this low frequency vibration, the dynamic characteristics of the mechanical system end up falling.